U.S. patent application number 13/346994 was filed with the patent office on 2012-07-19 for optical writing head and image forming apparatus.
This patent application is currently assigned to CANON KABUSHIKI KAISHA. Invention is credited to Kazuya Nobayashi.
Application Number | 20120182371 13/346994 |
Document ID | / |
Family ID | 46490471 |
Filed Date | 2012-07-19 |
United States Patent
Application |
20120182371 |
Kind Code |
A1 |
Nobayashi; Kazuya |
July 19, 2012 |
OPTICAL WRITING HEAD AND IMAGE FORMING APPARATUS
Abstract
An optical writing head allows the formation of a light
collective spot with a small diameter and the acquisition of a deep
focal depth to go together. An optical writing head includes a
light emitting element array in which a plurality of light emitting
elements is arranged, and a lens system including a lens array
configured to concentrate luminous flux radiated from the light
emitting element to a predetermined image plane, in which the lens
system is telecentric on the image side and satisfies the following
conditional expression, where a wavelength at which luminous flux
radiated from the light emitting element has a peak light intensity
is .lamda..sub.0, axial chromatic aberration of a wavelength having
a light intensity approximately 0.81 times the peak light intensity
is .DELTA..sub.sk, and the numerical aperture of the lens system on
the image side is NA. .DELTA. sk > .lamda. 0 2 1 NA ( .lamda. 0
) 2 ##EQU00001##
Inventors: |
Nobayashi; Kazuya; (Tokyo,
JP) |
Assignee: |
CANON KABUSHIKI KAISHA
Tokyo
JP
|
Family ID: |
46490471 |
Appl. No.: |
13/346994 |
Filed: |
January 10, 2012 |
Current U.S.
Class: |
347/224 |
Current CPC
Class: |
G02B 27/0043 20130101;
B41J 2/451 20130101; G02B 27/4227 20130101; G02B 27/4211 20130101;
B41J 2/47 20130101; G02B 27/0056 20130101 |
Class at
Publication: |
347/224 |
International
Class: |
B41J 2/435 20060101
B41J002/435 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 14, 2011 |
JP |
2011-005818 |
Claims
1. An optical writing head comprising: a light emitting element
array in which a plurality of light emitting elements is arranged;
and a lens system including a lens array configured to concentrate
luminous flux radiated from the light emitting element to a
predetermined image plane, wherein the lens system is telecentric
on the image side and satisfies a conditional expression .DELTA. sk
> .lamda. 0 2 1 NA ( .lamda. 0 ) 2 ##EQU00005## where
.lamda..sub.0 is a wavelength at which luminous flux radiated from
the light emitting element has a peak light intensity,
.DELTA..sub.sk is axial chromatic aberration of a wavelength having
a light intensity approximately 0.81 times the peak light
intensity, and NA is the numerical aperture of the lens system on
the image side.
2. The optical writing head according to claim 1, wherein the lens
system is comprised of a plurality of diffractive optical
elements.
3. The optical writing head according to claim 1, wherein the lens
system is telecentric on the image side.
4. The optical writing head according to claim 1, wherein the
absolute value of the lateral magnification in the lens system is
greater than 1.
5. The optical writing head according to claim 1, wherein the
emission angle of the luminous flux radiated from the light
emitting element is equal to the aperture angle of the lens system
on the object side.
6. An image forming apparatus comprising the optical writing head
according to claim 1 and a photosensitive portion which the optical
writing head irradiates with light to form a latent image.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] Embodiments of the present invention relate to an optical
writing head and an image forming apparatus and, in particular, to
an optical writing head which is used for an electrophotographic
copying machine, a printer, and a facsimile machine and forms an
imaging spot by a lens array projecting beams from a plurality of
light emitting elements onto a light receiving surface and an image
forming apparatus using the optical writing head.
[0003] 2. Description of the Related Art
[0004] Until now, an optical writing head used for
electrophotographic copying machine and other devices includes a
light emitting element array in which a plurality of light emitting
elements such as a light emitting diode (LED) is arranged, and a
lens array in which a plurality of lenses is arranged in parallel
between photosensitive drums which are image carriers.
[0005] Luminous flux modulated according to an image signal is
emitted from a light emitting element and concentrated by the lens
array at the surface of the photosensitive drum in a spot shape to
record an image.
[0006] Such an optical writing head is required to perform
higher-definition printing.
[0007] The photosensitive drum is cylindrical and is rotated around
the axis of the cylinder as a rotation axis. The rotation axis
deviates from the center axis of the photosensitive drum due to a
production error and an installation error at the time of
assembly.
[0008] For this reason, an image plane moves back and forth in the
optical-axis direction according to the rotation of the
photosensitive drum. As a result, a spot position and a spot
diameter formed on the surface of the photosensitive drum are
varied according to the rotation of the photosensitive drum, which
produces uneven density and color change on the recorded image.
[0009] Hitherto, U.S. Patent Application Publication
US2009/0086328A1 discusses a unit capable of reducing a change in a
spot position caused by an image plane moving back and forth in the
optical-axis direction by making an image side of a lens array
telecentric. According to such a configuration, a principal ray is
parallel to the optical axis, so that a change in a spot position
is smaller even if the image plane moves back and forth and a
change in a spot diameter may be reduced within the focal depth of
the lens array.
[0010] In order for a conventional method to perform a
higher-definition printing, it is required to increase the
numerical aperture on the image side (image-side NA) and form a
light collective spot to be small in diameter on the surface of a
photosensitive drum.
[0011] On the other hand, the increase of the image-side NA of the
lens array decreases the focal depth of the lens array, which
narrows the tolerance of a change in position of the image
plane.
[0012] Thus, in order to perform a higher-definition printing, the
photosensitive drum needs to be accurately installed at the time of
production and assembly. This increases the cost of an optical
writing head and a photosensitive drum.
[0013] Accordingly, the above situation demands an optical writing
head in which a light collective spot of a small diameter and a
deep focal depth go together.
SUMMARY OF THE INVENTION
[0014] One disclosed aspect of the embodiments provides an optical
writing head and an image forming apparatus in which the formation
of a light collective spot of a small diameter and the acquisition
of a deep focal depth go together.
[0015] According to an aspect of the embodiments, an optical
writing head includes a light emitting element array in which a
plurality of light emitting elements is arranged, and a lens system
including a lens array configured to concentrate luminous flux
radiated from the light emitting element to a predetermined image
plane, in which the lens system is telecentric on the image side
and satisfies the following conditional expression where a
wavelength at which luminous flux radiated from the light emitting
element has a peak light intensity is .lamda..sub.0, axial
chromatic aberration of a wavelength having a light intensity
approximately 0.81 times the peak light intensity is
.DELTA..sub.sk, and the numerical aperture of the lens system on
the image side is NA.
.DELTA. sk > .lamda. 0 2 1 NA ( .lamda. 0 ) 2 ##EQU00002##
[0016] According to another aspect of the embodiments, an image
forming apparatus includes the above optical writing head and a
photosensitive portion which the optical writing head irradiates
with light to form a latent image thereon.
[0017] One disclosed aspect of the embodiments provides an optical
writing head and an image forming apparatus which allow satisfying
both of the formation of a light collective spot small in diameter
and the acquisition of a deep focal depth.
[0018] Further features and aspects of the embodiments will become
apparent from the following detailed description of exemplary
embodiments with reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] The accompanying drawings, which are incorporated in and
constitute a part of the specification, illustrate exemplary
embodiments, features, and aspects of the embodiments and, together
with the description, serve to explain the principles of the
embodiments.
[0020] FIG. 1 is a schematic cross sectional view illustrating a
configuration of an optical writing head according to an exemplary
embodiment.
[0021] FIG. 2 is a schematic cross sectional view illustrating a
configuration of a part of the optical writing head according to an
exemplary embodiment.
[0022] FIGS. 3A, 3B, and 3C are charts describing the principle of
the optical writing head according to an exemplary embodiment.
[0023] FIG. 4 is a chart illustrating the center intensity of a
point image of the light emitting element and the center intensity
of a point image at a peak wavelength of the light emitting element
to describe the effect of the optical writing head according to an
exemplary embodiment.
[0024] FIG. 5 is a chart describing the effect of the optical
writing head according to an exemplary embodiment.
[0025] FIGS. 6A, 6B, and 6C are charts describing an example of a
configuration of the optical writing head according to the first
exemplary embodiment.
[0026] FIGS. 7A, 7B, and 7C are charts describing an example of a
configuration of the optical writing head according to the second
exemplary embodiment.
[0027] FIGS. 8A, 8B, and 8C are charts describing the optical
writing head using an optical system according to the second
exemplary embodiment.
DESCRIPTION OF THE EMBODIMENTS
[0028] Various exemplary embodiments, features, and aspects of the
embodiments will be described in detail below with reference to the
drawings.
[0029] The configuration of an optical writing head according to an
exemplary embodiment is described below with reference to FIG.
1.
[0030] FIG. 1 is a schematic cross sectional view of an optical
writing head 100 taken along a plane parallel to the main scanning
direction. In FIG. 1, a light emitting element 101 is arranged on a
substrate 105. A plurality of the light emitting elements 101 is
arranged on a substrate to form a light emitting element array. The
light emitting element 101 may be composed of a light emitting
diode (LED) or an organic electroluminescence display (EL), for
example. A lens array 102 includes a plurality of lens arranged in
parallel and concentrates luminous flux radiated from the light
emitting element onto a predetermined image plane.
[0031] Luminous flux radiated from the light emitting element 101
is concentrated by the lens array 102 to form a spot on the surface
of the photosensitive drum 103.
[0032] The lens array 102 has an image-side telecentric
configuration in which a principal ray is parallel to an optical
axis on the image side. For example, FIG. 2 illustrates a schematic
diagram of a part of an optical writing head 100.
[0033] In FIG. 2, the light emitting elements 101 are arranged in
an array shape on a light source surface (object surface) 201.
[0034] A diaphragm plate 202 and the lens array 102 are disposed
between the light source surface 201 and a photosensitive-member
surface (image plane) 203. A distance between the diaphragm 202 and
the object-side principal point of the lens array 102 is taken as a
focal length of the lens array 102. Such an arrangement makes the
lens array 102 image-side telecentric. A light emission spectrum
radiated from the light emitting element 101 has a finite wave
length width.
[0035] The principle of the optical writing head according to the
exemplary embodiment is described below with reference to FIGS. 3A,
3B, and 3C. FIG. 3A illustrates an example of a light emission
spectrum of the light emitting element 101. In FIG. 3A, the
abscissa is wavelength and the ordinate is light intensity.
[0036] The light emitting element 101 has a light intensity 301B at
a wavelength 301A, alight intensity 302B at wavelengths 302A and
304A, and a light intensity 303B at wavelengths 303A and 305A.
[0037] When the light radiated from the light emitting element 101
is concentrated by the lens array 102, images are formed at
different positions for each wavelength due to axial chromatic
aberration.
[0038] FIG. 3B is a chart illustrating the center intensity of
point images (point-image center intensity) of the wavelengths 301A
to 305A at each defocus position.
[0039] In FIG. 3B, the abscissa represents defocus with a paraxial
image plane of the wavelength 301A as a reference and the ordinate
represents a point-image center intensity at each defocus
position.
[0040] The point images of each wavelength have the maximum
intensity at different defocus positions due to axial chromatic
aberration. The light emitting element 101 emits light including a
plurality of wavelengths in addition to the wavelengths 301A to
305A. For this reason, the point-image center intensity of the
light emitting element 101 may be represented by integrating the
point image of each wavelength.
[0041] A solid line 306 in FIG. 3C indicates the point-image center
intensity of the light emitting element 101 obtained by integrating
the point image of each wavelength. A dotted line 307 in FIG. 3C
indicates the maximum intensity of the point image of each
wavelength.
[0042] In FIG. 3C, the abscissa represents defocus with a paraxial
image plane of the wavelength 301A as a reference and the ordinate
represents the intensity normalized at a defocus=0 for the purpose
of comparison.
[0043] From FIG. 3C, it may be seen that the solid line 306 and the
dotted line 307 show almost equal defocus characteristics.
[0044] Therefore, a change in the center intensity of point image
of the light emitting element 101 per unit defocus may be
represented by a change in the maximum intensity of point image of
each wavelength per unit defocus.
[0045] The smaller a change in the center intensity of point image
per unit defocus, the smaller a change in spot diameter at each
defocus position and the deeper the focal depth.
[0046] The center intensity of point image of the light emitting
element 101 has the defocus characteristic which is almost equal to
the maximum intensity of point image of each wavelength, so that
the axial chromatic aberration of the lens array 102 is increased
to reduce a change in the center intensity of point image per unit
defocus and deepen the focal depth.
[0047] Thereby, the focal depth may be deepened without reducing
the image-side NA of the lens array 102, so that the small spot
diameter and the deep focal depth may go together.
[0048] Next, a range is described below which has a great effect in
deepening the focal depth.
[0049] FIG. 4 illustrates the center intensity of point image of
the light emitting element 101 and the center intensity of point
image at a peak wavelength .lamda.0 at which the luminous flux
emitted from the light emitting element 101 has a peak light
intensity.
[0050] In FIG. 4, the abscissa represents defocus with a paraxial
image plane of the peak wavelength .lamda.0 of the light emitting
element 101 as a reference and the ordinate represents the
intensity normalized at a defocus=0.
[0051] A solid line 401 in FIG. 4 indicates the point-image center
intensity of the light emitting element 101. A dotted line 402 in
FIG. 4 indicates the point-image center intensity at the peak
wavelength .lamda.0.
[0052] In FIG. 4, reference numeral 403 denotes the point-image
center intensity which is approximately 0.81 times a point-image
center intensity at a defocus=0.
[0053] At a defocus 404, the solid line 401 assumes the point-image
center intensity 403. At a defocus 405, the dotted line 402 assumes
the point-image center intensity 403.
[0054] It is generally said that a change in an image of an optical
system is small in an area where the point-image center intensity
is approximately 0.81 times or less a point-image center intensity
at a defocus=0. Therefore, focal depth may be represented by
defocus with the point-image center intensity 403.
[0055] In other words, the defocus 405 represents the focal depth
of the lens array 102 determined by the image-side numerical
aperture NA. The defocus 404 represents the focal depth of the
optical writing head 100 according to one disclosed aspect of the
embodiments.
[0056] If the defocus 405 is taken as z.sub.0, z.sub.0 is
represented by the following conditional expression 1. In the
expression 1, NA refers to the image-side numerical aperture of the
lens array 102.
z 0 = .lamda. 0 2 1 NA 2 Expression 1 ##EQU00003##
[0057] The defocus 404 assumes axial chromatic aberration .DELTA.sk
of a wavelength having a light intensity which is approximately
0.81 times peak light intensity with a light emission spectrum of
the light emitting element 101.
[0058] If the defocus position .DELTA..sub.sk of the solid line 401
has a value greater than the defocus position .DELTA..sub.0 of the
dotted line 402, the focal depth may be deepened irrespective of
the image-side NA of the lens array 102.
[0059] Therefore, the focal depth may be deepened irrespective of
the image-side NA of the lens array 102 by satisfying the following
conditional expression 2. Thereby, the small spot diameter and the
deep focal depth may go together.
.DELTA. sk > .lamda. 0 2 1 NA ( .lamda. 0 ) 2 Expression 2
##EQU00004##
[0060] In FIG. 5, the point-image center intensity of the peak
wavelength of the light emitting element 101 is represented by a
solid line 501. The point-image center intensity of the light
emitting element 101 when the conditional expression 2 is satisfied
is represented by a dotted line 502. The point-image center
intensity of the light emitting element 101 obtained when equality
sign is had in the conditional expression 2 is represented by an
alternate long and short dash line 503.
[0061] In FIG. 5, the ordinate and the abscissa are similar to
those in FIG. 4.
[0062] The alternate long and short dash line 503 draws a similar
curve as the solid line 501 and an effect of deepening the focal
depth is small.
[0063] On the other hand, with respect to the dotted line 502, a
change in the point-image center intensity per unit defocus is
smaller than the solid line 501, which enables deepening of the
focal depth.
[0064] As described above, the focal depth may be made deeper than
that determined by the image-side NA of the lens array 102 by
satisfying the conditional expression 2.
[0065] According to one disclosed aspect of the embodiments, the
image-side telecentric is not limited to a case where the principal
ray incident on the image plane is completely parallel to an
optical axis. The image-side telecentric includes the case where an
angle made by the optical axis and the principal ray incident on
the photosensitive drum 103 which is the image plane, is within
.+-.1%.
[0066] In one disclosed aspect of the embodiments, an image forming
apparatus may be formed such that a photosensitive portion is
irradiated with light from the optical writing head to form a
latent image using the optical writing head.
[0067] An exemplary embodiment is described below.
[0068] An example of an optical writing head to which one disclosed
aspect of the embodiments is applied is described as a first
exemplary embodiment. FIG. 6A is a graph illustrating the light
emission spectrum of the light emitting element 101. The abscissa
is wavelength and the ordinate is light intensity.
[0069] The light emitting element 101 has the light emission
spectrum with a full width at half maximum of 40 nm.
[0070] A wavelength 601 is a peak wavelength .lamda..sub.0 of the
light emitting element 101 and .lamda..sub.0=546 .mu.m in the
present exemplary embodiment.
[0071] Wave lengths 602 and 603 are wavelengths .lamda..sub.1 and
.lamda..sub.2 with a light intensity 604 which is approximately
0.81 times a peak intensity 605. The wavelength .lamda..sub.1=535
nm and the wavelength .lamda..sub.2=558 nm. FIG. 6B illustrates a
main scanning cross section of the optical system corresponding to
a single lens array 102 according to the first exemplary embodiment
including a doublet lens comprised of a first lens which is a crown
lens and a second lens which is a flint lens. A diaphragm plate 607
is arranged at a focus of a combination lens system made of the
first and second lenses on the object side to make the image side
telecentric.
[0072] According to the present exemplary embodiment, an axial
chromatic aberration .lamda..sub.sk=-0.0536 mm at the wavelength
.lamda..sub.1, and an axial chromatic aberration
.DELTA..sub.sk=0.0540 mm at the wavelength .lamda..sub.2. The right
side member of the conditional expression 2 is 0.0486 mm using the
aperture angle on the image side and the peak wavelength, which
satisfies the conditional expression 2.
[0073] FIG. 6C illustrates a point-image center intensity and a
defocus characteristic in the present exemplary embodiment. The
abscissa represents defocus with a paraxial image plane of the
wavelength .DELTA..sub.0 as a reference and the ordinate represents
the point-image center intensity. A solid line 612 illustrates a
point-image center intensity in the present exemplary embodiment. A
dotted line 613 illustrates a point-image center intensity at the
wavelength .lamda..sub.0.
[0074] From FIG. 6C, it may be seen that in the solid line 612 a
change in the point-image center intensity per unit defocus is
smaller than the dotted line 613 and with the configuration of the
present exemplary embodiment, a deep focal depth is acquired.
[0075] The spectrum width of a general LED is substantially 40 nm.
For this reason, a difference between a wavelength with a light
intensity approximately 0.81 times a peak light intensity, and a
peak wavelength is substantially 10 nm. A conventional lens array
uses a material whose Abbe number is great to lessen the influence
of chromatic aberration.
[0076] If a resinous positive lens of polycarbonate whose
refractive index dispersion is great (Abbe number is small) is
used, the axial chromatic aberration thereof is as small as 0.03 mm
to 0.04 mm, which is difficult to satisfy the conditional
expression 2.
[0077] As described in the present exemplary embodiment, the
doublet lens in which the crown lens and the flint lens with
different refractive indexes are combined, is effective to acquire
the axial chromatic aberration satisfying the conditional
expression 2.
[0078] The numerical data of the present exemplary embodiment are
shown below. In the order from a light source surface 606 to a
photosensitive surface 611, r2, r3, . . . (mm) represent curvature
radii of each optical surface, d1, d2, . . . (mm) represent
distances between optical surfaces, and refractive indexes n1, n2,
. . . represent each transparent medium.
[0079] The refractive index of each transparent medium is
represented by using the dispersion formula of the following
conditional expression 3.
n.sup.2=C.sub.1+C.sub.2.lamda..sup.2+C.sub.3.lamda..sup.-2+C.sub.4.lamda-
..sup.-4+C.sub.5.lamda..sup.-6+C.sub.6.lamda..sup.-8 Expression
3
[0080] The coefficients of the refractive indexes n1 and n2 have
the following values.
n1: C.sub.1=2.633127 C.sub.2=-7.937823E-2 C.sub.3=-1.734506E-1
C.sub.4=8.609268E-2 C.sub.5=-1.617892E-2 C.sub.6=1.128933E-3 n2:
C.sub.1=2.399964 C.sub.2=-8.308636E-2 C.sub.3=-1.919569E-1
C.sub.4=8.720608E-2 C.sub.5=-1.666411E-2 C.sub.6=1.169519E-3
r1=.infin.(object surface) d1=4 r2=0.39549 d2=0.25 n1 r3=-0.64735
d3=0.06 n2 r4=0.33311 d4=3 r5=.infin.(image plane)
[0081] Aperture angle on the object side (half angle)=0.075 rad
Aperture angle on the image side (half angle)=0.075 rad Lateral
magnification=-1.0
[0082] A second exemplary embodiment is different from the first
exemplary embodiment in that a plane with a positive power is
formed by a diffractive optical element (DOE).
[0083] A power .phi..sub.1 of the DOE at the wavelength
.lamda..sub.1 may be expressed by the following equation when power
is .phi..sub.0 at a wavelength .lamda..sub.0:
(.phi..sub.1=(.lamda..sub.1/.lamda..sub.0).times..phi..sub.0,
and a change in power per unit wavelength is great. This provides a
great axial chromatic aberration.
[0084] FIG. 7A is a graph illustrating the light emission spectrum
of the light emitting element 101. The abscissa is wavelength and
the ordinate is light intensity.
[0085] The light emitting element 101 has the light emission
spectrum with a full width at half maximum of 40 nm. A wavelength
701 is a peak wavelength .lamda..sub.0 of the light emitting
element 101 and .lamda..sub.0=680 .mu.m in the present exemplary
embodiment.
[0086] Wave lengths 702 and 703 are wavelengths .lamda..sub.1 and
.lamda..sub.2 with a light intensity 704 approximately 0.81 times a
peak intensity 705. The wavelength .lamda..sub.1=669 nm and the
wavelength .lamda..sub.2=691 nm.
[0087] FIG. 7B illustrates a main scanning cross section of the
optical system corresponding to a single lens array 102 according
to the second exemplary embodiment.
[0088] A first lens is formed of the DOE including a front side
face 710 and a rear side face 711. A light source surface 706 is
arranged at a focus of a front side lens of the first lens 708 on
the object side.
[0089] A distance between the front side face 710 and the rear side
face 711 of the first lens 708 is made equal to a value in which
the sum of the focal lengths of the front side face 710 and the
rear side face 711 is divided by the refractive index of the first
lens so that the objet side and the image side are made
telecentric.
[0090] The present exemplary embodiment has an axial chromatic
aberration .lamda..sub.sk=0.0718 mm at the wavelength .lamda..sub.1
and an axial chromatic aberration .DELTA.sk=-0.0718 mm at the
wavelength .lamda..sub.2. The right side member of the conditional
expression 3 is 0.034 mm using the aperture angle on the image side
and the peak wavelength and satisfies the conditional expression
3.
[0091] FIG. 7C illustrates a point-image center intensity and a
defocus characteristic in the present exemplary embodiment. The
abscissa represents defocus with a paraxial image plane of the
wavelength .DELTA..sub.0 as a reference and the ordinate represents
the point-image center intensity.
[0092] A solid line 712 illustrates a point-image center intensity
in the present exemplary embodiment. A dotted line 713 illustrates
a point-image center intensity.
[0093] From FIG. 7C, it may be seen that a change in the
point-image center intensity per unit defocus is smaller in the
solid line 712 than the dotted line 713, and in the configuration
of the present exemplary embodiment, a deep focal depth may be
acquired. In the present exemplary embodiment, the DOE is used for
the optical surface to generate a great axial chromatic aberration
and acquire a deeper focal depth.
[0094] In the present exemplary embodiment, not only the image side
but also the objet side are made telecentric. The objet side is
also made telecentric to inhibit generation of the unevenness of
the light amount and form a uniform latent image on a
photosensitive member.
[0095] The lens array 102 in the present exemplary embodiment is an
inverted magnifying optical system with a lateral magnification of
-1.2 times. The absolute value of the lateral magnification is made
greater than 1 to make the axial chromatic aberration greater and
deepen the focal depth.
[0096] FIG. 8A illustrates a schematic diagram of the optical
writing head 100 using a magnifying optical system. The arrangement
of the light emitting element 101 is determined in consideration of
the amount of magnification in which an interval 801 between the
light emitting elements 101 is magnified by the magnification of
the lens array 102, so that one column of images may be formed on
the photosensitive drum 103.
[0097] In the present exemplary embodiment, the lens array 102 uses
the inverted magnifying optical system, but may use an inverted
reducing optical system.
[0098] FIGS. 8B and 8C illustrate schematic diagrams of the optical
writing head 100 using a reducing optical system. In the optical
writing head 100 illustrated in FIGS. 8B and 8C, a plurality of
columns of the lens arrays 102 is arrayed also in the sub-scanning
direction. FIGS. 8B and 8C illustrate schematic diagrams of columns
different in the sub-scanning direction.
[0099] In such a configuration, the lens array 102 is shifted and
arranged in parallel in the main scanning direction between columns
different in the sub-scanning direction.
[0100] The light emitting element 101 is modified in
synchronization with the rotation of the photosensitive drum 103 to
form one column of images on the photosensitive drum 103.
[0101] In the reducing optical system, axial chromatic aberration
is smaller than the magnifying optical system, however, the size of
an image may be smaller than the magnifying optical system and a
latent image with a small diameter may be formed on the
photosensitive drum 103.
[0102] Although the emission angle of the light emitting element
101 is not limited in the present exemplary embodiment, the
emission angle (half angle) of the light emitting element 101 is
desirably 0.012 radian, which is equal to the aperture angle of the
lens array 102 on the object side.
[0103] The emission angle of the light emitting element 101 is made
equal to the aperture angle on the object side of the lens array
102 to eliminate the need for a diaphragm plate in the lens array
102 and light flux emitted from the light emitting element 101 may
be efficiently guided to a photosensitive surface 709.
[0104] The numerical data of the present exemplary embodiment are
shown below.
[0105] In the order from a light source surface 706 to a
photosensitive surface 709, f1, f2, . . . represent focal lengths
of each optical surface, d1, d2, . . . represent distances (mm)
between optical surfaces, and refractive index n1 represents a
transparent medium. The refractive index of each transparent medium
is represented by using the dispersion formula of the conditional
expression 3. Coefficients of the refractive index n1 have the
following values.
n1: C.sub.1=2.399964 C.sub.2=-8.308636E-2 C.sub.3=-1.919569E-1
C.sub.4=8.720608E-2 C.sub.5=-1.666411E-2 C.sub.6=1.169519E-3
f1=.infin.(object surface) d1=1.6667 f2=1.6667 d2=5.4525 n1 f3=2
d3=2 f4=.infin.(image plane) Aperture angle on the object side
(half angle)=0.012 rad Aperture angle on the image side (half
angle)=0.01 rad Lateral magnification=-1.2
[0106] While the embodiment has been described with reference to
exemplary embodiments, it is to be understood that the invention is
not limited to the disclosed exemplary embodiments. The scope of
the following claims is to be accorded the broadest interpretation
so as to encompass all modifications, equivalent structures, and
functions.
[0107] This application claims priority from Japanese Patent
Application No. 2011-005818 filed Jan. 14, 2011, which is hereby
incorporated by reference herein in its entirety.
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